Coupled circuits



June 21, 1938. A. w. BARBER COUPLED CIRCUITS Filed Feb. 3, 1936 INVENTOR Patented June 21, 1938 UNITED STATES gums PATENT OF FlCE 15 Claims.

In my copending application for Letters Pat-- ent dated January 2'7, 1936 and entitled Coupled circuit systems Serial No. 61,458, I have shown,

circuit coupling means consisting of the grid to cathode capacity of a thermionic vacuum tube. I have also shown how the grid to cathode capacity and hence the coupling may be controlled by varying the grid bias in the coupling tube. Further, I have shown how this control may be effected from the rectified signal, and hence the coupling and amplifier response made to automatically vary with the strength of the, received signal. My present invention is a system devised for thesame purpose but showing a more sensitive control characteristic.

In radio and carrier wave amplifiers the interstage selective means usually consists of pairs of tuned circuits. If the two circuits comprising each pair are coupled together by varying amounts, three general types of response are possible. With less than a particular coupling called critical, the response of two circuits tuned to the same frequency is a single peak but the amplitude of transmission becomes less as the coupling is decreased. At critical coupling the response is still a single peak but with some broadening of the peak and a maximum amplitude of transmission. If the coupling is increased above critical, the amplitude of response remains the same but asecond peak appears and as the coupling is increased, the frequency of the second response moves further away from the tuning frequency. Due to receiving conditions, it is often necessary to use the maximum obtainable selectivity in a radio receiver which 7 requires critical coupling or less between interstage circuits. However, under this condition most systems produce serious attenuation of modulationfrequencies above 3000 cycles. In some receivers manual means have been provided for increasing coupling and hence broadening response when receiving conditions permit. Since the possibility of using broadened response usually occurs on strong stations, I have found it possible to control coupling automatically as a function of signal strength and thus do away with manual control.

In my copending application, I have shown how automatic coupling may be provided in the control of thermionic vacuum tube input capacity. The dynamic input capacity of a thermionic vacuum; tube depends mainly on the grid to plate capacity and grid to plate gain of the tube. Since the tube gainmay be controlled by means of its grid bias, the input capacity may also be controlled by grid bias changes. If the bias is derived, in part, from the rectified signal traversing the receiver or amplifier, the couplingand hence band-pass characteristics may be made to automatically follow the received signal strength. My present invention makes use of this same a controlled tube input capacity but the sensitivity of the control is greatly increased by using resonant or partly resonant coupling means. The tube input capacity is used in conjunction with inductive coupling means and by means of series resonance, the coupling is made to vary more rapidly as a function of tube gain than in the simple common impedance type of coupling.

The appended claims set forth, in particular,

the novel features to be found in this invention. The following description, however, when taken in connection with the drawing, will serve to set forth the theory and mode of operation of my invention.

In the drawing,

Fig. 1 shows the circuit of a radio receiver, up to and including the second detector, embodying my invention.

Fig. 2 shows the equivalent circuit of two tuned circuits coupled by my present system.

Fig. 3 shows a circuit equivalent to 'Fig. 2.

'Fig. 4 shows curves characteristic of the operation of my invention.

In Fig. 1, I have shown a superheterodyne receiver exclusive of the audio amplifier and loud speaker. first detector I, intermediate frequency amplifiers 2 and 3, second detector 4, oscillator 5 and coupling tubes 6 and l. The first detector-oscillator The tube complement consists of a circuits are conventional. +EB denotes points of connection of plate voltage supply and +En denotes points of connection of screen voltage supply. Between plate 8 of the first detector l and the plate voltage supply is connected a tuned circuit consisting of coil 9 tuned by condenser l0. Coil 9 and condenser form the tuned primary of an interstage transformer. Coil H tuned by condenser 12 forms the tuned secondary of the interstage transformer feeding grid l3 of the following amplifier tube 2. Coupling between coils 9 and II is accomplished by means of coil l4 magnetically coupled to primary coil and coil l magnetically coupled to secondary coil H and an external series impedance thru condenser I 6. The voltage induced in coil l4 from coil .9 causesa current to fiow in coil ill of a magnitude depending on the impedances of coils I4 and I 5 and the impedance between the lower end of coil. l4 and ground. This impedance is condenser [6 in series with the inputor grid to cathode dynamic impedance of tube *6. This input impedanceis essentially a capacity reactance. Condenser i6 is shown as a'blockingcondenser having -a capacitylarge compared 'to the tube input capacity. The controlled current flowing in the series circuit flows thru 'coil I5 inducing a voltagelin coil'll whichis afunction of the current.

The equivalent "circuit-of this coupling system isshown in Fig. 2 where the primary-circuit consists-of inductance Lrtunedby condenser 01, and the secondary circuit consists'of inductance L4 tuned byrcondenserC-i. Coupling between L1 and L4 is:accomplished by thecoupling link consisting-of inductances L2 and L3 and condenser Czs .all in series. L2 iscmagnetically'coupled to L1 .and Lsis .magneticallycoupled toL4. L1 induces a voltageinLz and a'current fiowsin the L2, L3, C23 seriescircuit. Since-Czsis variable the current depends on .the value ofCza and the'voltage induced in Lil-depends on thiscurrent flowing in L3. It should be.noted that if at the operating frequency, C23 resonates L2 and L3 in series, a series resonant circuit is produced in which the series current increases very rapidly as C23 is varied. C23 represents the grid to-cathode capacity of tube 6 or 1 in Fig. 1.

Fig. 4 shows aplot of reactance against bias where bias designates the variable component of the bias'appliedto the-grids of thecoupling tubes .6 or 1. Thehorizontal axis may also be condenser capacity inorderzto explain Fig. 2. Luis the reactance of Lz-and L3 (or'coil and coil H3) in series.

is the reactance of C23 (orthe input capacity of tubes 6 or 1) as the'capacity is'varied. The reactance of the series circuit is then shown by the curve 1 Leo The increase in sensitivity of the tuning eiiect over the simpleself-reactance coupling is shown by comparingthelast two curves. Starting with a bias or condensersetting athe bias-or capacity alone mustbevaried to point 0 in order to halve the series coupling reactance. However, with the inductance added the reactance is halved in going from a to b on the Leo-a curve. The apparent sharpening of control becomes greateras the series resonant point is ap- -a triode although I prefer'one.

proached and by proper choice of circuit elements a wide range of control slopes may be obtained.

Fig. 3 shows a circuit equivalent to Fig. 2 in terms of mutual and leakage inductance all reduced to unity turns ratio. M12 is the mutual inductance between L1 and L2 and m4 is the mutual inductance between L3 and L4. This circuit shows that the series resonant coupling impedance is not accurately L2, L3 and C23 but is LziMiz plus L3iM34 and C23 in series. Looking at the system thus from the standpoint of an equivalent circuit, C23 is equivalent to a high side coupling reactance and resonates with the leakage reactance of L2 and In to provide a sharp coupling eiTect.

Returning to Fig. 1 the input capacity of tube 6 is equivalent to condenser C23 of Figs. 2, 3 and 4. This input capacity depends on the tube characteristics, operating voltages, plate load and grid to plate capacity. Actually it depends on the-tube grid to plate gain and grid to plate capacity. With no external applied bias the tube gain'depends on the tube characteristics, plate voltage EB, cathode bias E0 and plate load resistor I1. While I .have found the simple resistor I! to be a satisfactory load, a complex impedance may be used. Thetube is not limited to Tube'fi is shown having a cathode 18, heated'by means not shown, a control grid l9 and a plate 20. The grid to plate capacity 2! maybe taken to represent the internal tube capacity plus external added capacity. The initial bias is provided by battery E0 in the cathode circuit. The control bias is supplied thru the resistor 22. Condensers 23 and Marc by-pass condensers. The circuit is designed so that With no external applied bias thru resistors 22 and 25, the circuit coupling is the minimum desired which will'in general be-criti- .calcoupling, for maximum selectivity and gain.

If an external positive bias is applied to grid IS,

the gain of tube 6 is increased increasing its input capacity. Increasing the input capacity increases the current flowing in the coupling circuit increasing the coupling. As the coupling is increased over critical, a-double peak appears in the response :producing a band-pass characteristic.

The same coupling system is shown between tubes 2 and 3 that was shown and described between tubes 1 and 2. Tube 3-feeds a tuned output circuit consisting. of inductance'26 tuned by condenser 21. The double diode second detector '4.is fedfrom coils 28 and 29 magnetically coupled to coil 26. The voltageacross coil 28 is impressed on the diode formed by plate 30 and cathode 3| thru'the load by-pass condenser 32. The load resistor 33 develops a rectified current voltage drop which maybe used for automatic volume control of tubes 2 and '3 by applying the drop to grids l3 thruthe filter consisting of resistors 34 and .35 and the condensers 36 and .31. The audio voltage for actuating the audio amplifier and speaker may also be obtained from the drop across resistor 33 or a separate rectifier may be employed.

The voltage across coil 29 is applied to the diode consisting of plate 38 and cathode '39 thru the by-pass condenser 40. The rectified output is a drop thru resistor 4| and the direct current component is filtered out by means of resistors 25 and condensers 23. This direct current component is proportional to the signal output from tube 3 and is a function of the signal picked up by antenna A. Since the cathode 39 end of load resistor-4| becomesmore positive thegreater the the system provides a response which is a function of the received signal and the stronger the received signal the wider the response band and the better the amplifier fidelity.

While not in any way intended to limit the scope of my invention, I have found the following constants to give the indicated expansion:

Tube or I mutual conductance at normal Condenser It to tune coil 9 to 547 k. c. Condenser, it to tune coil H to 547 k. c.

Coupling between coils 9 and I4 and between coils ii and I5 close.

Coupling with no external grid bias very nearly critical giving single response peak, with plate current zero.

With external bias to give plate current 5 ma. over coup-ling produced giving two peaks separated 30 k. c. s 1

A set of typical results is here shown as peak frequencies for various coupling tube plate currents.

0 ma. 20 ma.

Peak 547 k. 0. Peck 547 c.

Peak 497 An eifective delayed expansion may be produced by increasing E0 beyond cut-off. Since no expansion will take place until the coupling tubes 6 and i draw plate current, the delay depends on the amount the bias E0 exceeds the cut-off bias of tubes 6 and 1. Suppose for instance 25 volts is the bias required for cut-off then if EC is made 40 volts it will take 15 volts of external bias to bring the 40 volts to a net 25 volt bias. If 15 volts on the rectifier corresponds to 1 millivolt on the antenna, no expansion will take place until the received signal exceeds 1 millivolt. As the net bias is reduced below 25 volts the coupling increases expanding the receiver response.

While I have shown my automatic coupling system applied to intermediate frequency amplifier circuits, it is by no means thus limited but may be applied to variably tuned stages such as in a tuned radio frequency receiver or in the preselector circuits of a superheterodyne. Many combinations are possible such as equal expansion control on two intermediate frequency stages; a control with a different delay control on a third intermediate stage and a control with a still different delay value on the pre-selector circuits.

While I have described only one system whereby my invention may be carried into effect and have pointed out a few possible variations, it will be apparent to one skilled in the art that many modifications are possible without departing from its spirit and scope as set forth in the appended claims.

What I claim is:

1. In a selective carrier wave amplifier, means for varying the selectivity 'of said amplifier comprising the combination of at least two thermionic Vacuum tube repeaters, a resonant circuit receiving the output of one of said repeaters, a second resonant circuit connected across the input of another of said repeaters, a series circuit comprising two coils, a condenser and the grid to cathode impedance of a third thermionic vacuum tube comprising at least a grid, cathode and plate wherein one of said coils is magnetically coupled to one of said resonant circuits and the other of said coils is magnetically coupled to the other of said resonant circuits whereby energy is transferred from one of said resonant circuits to the other wherein said impedance includes a capacity reactance component of greater magnitude than the combined inductive reactances of said two coils at the resonant frequency of at least one of said resonant circuits.

2. The combination as set forth in claim 1 and including means for supplying a bias to the grid of said third vacuum tube at least in part derived by rectifying the signal traversing said amplifier.

3. The combination as set forth in claim'l and including means for applying a bias to the grid of said third vacuum tube greater than that required for plate current cut-off for all received signals below a predetermined level.

4. In a radio receiver employing thermionic vacuum tube repeaters, an interstage coupling circuit comprising tuned input and output coils and a link circuit, a thermionic vacuum tube comprising at least a grid, cathode and plate, said link circuit including in series connection two coils and the grid to cathode impedance of the last said thermionic vacuum tube all connected in series, wherein said link is adapted to transfer energy between said tuned circuits by virtue of magnetic coupling between one of said coils and said output circuit, the coupling between other of said coils and said-input circuit and the series connected input impedance of the last said tube and including a condenser between gridand plate of the last said vacuum tube and a load resistor in series with said plate whereby the input impedance of the last said tube exhibits a capacity reactance greater than the inductive reactance of said two coilsin series at the resonant frequency of said tuned input coil.

5. In a selective carrier wave amplifier, means for varyingthe selectivity of said amplifier comprising the combination of at least two resonant circuits and a coupling link between said circuits, said link comprising in series at least one coil magnetically coupled to each of said resonant circuits and the grid to cathode capacity of a thermionic vacuum tube, means for causing said grid to cathode capacity to vary as a function of the amplitude of the signals traversing said amplifier when said signals are greater than a predetermined amplitude.

6. The combination as set forth in claim 5 wherein at the frequency of resonance of said resonant circuits the capacity reactance of said grid to cathode impedance is greater than the sum of the inductive reactances of said coils.

7.In the intermediate frequency amplifier of a superheterodyne radio receiver, the combina-' tion of a plurality of thermionic repeaters, at least two pairs of input and output resonant circuits associated with said repeaters, energy transfer means linking said input and output circuits in said pairs, in which said transfer means comprises series circuits consisting of a coil coupled to an input circuit and a second coil coupled to the output circuit of a pair and the input impedance of a thermionic vacuum tube and means for causing said input impedance to exhibit a capacity reactance component greater in magnitude than the sum of the inductive reactances of said two coils at the intermediate frequency of said receiver.

8. The combination as set forth in claim 7 and including means for applying a control bias to said vacuum tube which is proportional to the difierence between an initial fixed bias and a bias derived by rectification of the signal'traversing said amplifier.

9. The combination as set forth in claim 7 in which the said vacuum tube embodies a cathode, a grid and a plate, and is associated with an ex,- ternal circuit comprising a capacity connected between said grid and said plate and a resistor in series with said plate and means for applying a control bias to said grid equal to the difference between a fixed voltage and a voltage derived by rectification of the signal traversing said ampli- -fier.

10. In a carrier wave amplifier, the combination of at least two thermionic repeaters, a resonant circuit connected to the plate of one of said repeaters and a second resonant circuit connected to the grid of another of said repeaters, two coils and the input impedance of a thermionic vacuum tube connected in series, wherein one of said coils is magnetically coupled to said plate connected resonant circuit and the other of said coils is magnetically coupled to said grid connected resonant circuit, said vacuum tubeincluding a cathode, a grid and a plate, means associated with said vacuum tube including a capacity connected between said tube grid and said tube plate, a resistance connected between said tube plate and a source of positive potential, and external grid bias means comprising a source of fixed-bias and the direct current component of rectification of the signal traversing said amplifier acting in opposition wherein the input impedance of said vacuum tube exhibits a capacity reactance component of greater magnitude at all times than the inductive reactance of the two said coils in series at the resonant frequencyof one of said resonant circuits.

11. The combination as set forth in claim 10 wherein said fixed bias is greater than that required to produce plate current cut-off in said vacuum tube.

12. The combination as set forth in claim 10 wherein the capacity reactance component of said vacuum tube input impedance is a function i of the amplitude of the signal traversing said amplifier when said signal is greater than a predetermined value.

13. In a radio receiver embodying thermionic vacuum repeater tubes, the combination of at least one interstage coupling system comprising an input and an output tuned circuit and an intercircuit coupling impedance, said coupling impedance consisting of inductive means resonated to a frequency greater than the resonant frequency of either of said tuned circuits by an electrically controlled capacity means comprising the dynamic grid to cathode capacity of a thermionic Vacuum tube wherein said combination includes a condenser connected between grid and plate of the last said vacuum tube.

14. In a selective system, the combination of two tuned circuits and intercircuit coupling means comprising two coils and the dynamic grid to cathode impedance of a thermionic vacuum tube triode connected in series, wherein one of said coils is coupled to one of said tuned circuits and the other of said coils is coupled to the other of said tuned circuits and including a condenser connected between the grid and plate of said triode wherein said condenser has a reactance greater than the combined inductive reactances of said two coils.

15. In a carrier wave amplifier, automatic band-pass control means comprising a thermionic vacuum tube feeding a tuned circuit, a second tuned circuit feeding into a second thermionic vacuum tube, two coils and an electronically controlled capacity connected in series, wherein one of said c'oils is magnetically coupled to the first of said tuned circuits and the second of said coils is magnetically coupled to the second of said tuned circuits and wherein said electronically controlled capacity comprises the effective grid to cathode capacity of a third thermionic vacuum tube, said third tube comprising at least acathode, grid and plate, and further means comprising a condenser connected between plate and grid of said third tube and a load resistor in series with the plate of said third tube, means for applying a bias to the grid of said third tube at least in part derived from the rectification of signals traversing said amplifier wherein the minimum Value of said electronically controlled capacity resonates said two coils in series to a frequency greater than the resonant frequency of either of said tuned circuits.

ALFRED W. BARBER. 

